This invention relates to a detector for an electromagnetic flowmeter and in particular to a detector for an electromagnetic flowmeter for measuring the flow rate of electrically conductive fluid.
The flow rate signal E of an electromagnetic flowmeter is expressed according to JIS (Japan Industrial Standard) B7554-1984 as follows: EQU E=kBdv (1)
where
k: short circuit coefficient, PA1 B: magnetic flux density, PA1 d: inner diameter of piping (distance between detecting electrodes), and PA1 v: flow speed. PA1 r: radius varying between O and d/2, PA1 .DELTA.r: increment of radius, and PA1 v: flow speed (constant)
Usually, in a detector, there are disposed a pair of detecting electrodes 2 on the left and the right walls of a piping 1, whose cross-section is circular, for taking-out electric signals representing the flow rate, as indicated in FIG. 3. The distance between the electrodes is nearly equal to the inner diameter of the piping, i.e. the detector. There are disposed magnetic poles in the up and down direction perpendicular both to the axis connecting the 2 detecting electrodes and to the direction, along which the fluid 3 flows (axial direction of the piping), which poles are connected with a yoke 5. Further, there are mounted a pair of exciting coils 6 on the magnetic poles 4, which are excited by a rectangular AC current so that a magnetic field is applied to the fluid 3. These magnetic poles 4 may be replaced by a permanent magnet. In this case not only the exciting coils are unnecessary and the structure is simpler, but also this structure is more convenient for measurements of the flow rate of fluids, for which it is undesirable to be heated, because there is no heat production due to exciting current. This arrangement of the electrodes 2 and the magnetic poles 4 gives rise to an electric signal E expressed by Eq. (1), representing the flow rate depending on the speed of the fluid 3, according to the Faraday's electromagnetic induction law. The magnetic flux density distribution produced by such magnetic poles has a shape indicated in FIG. 4. By the method for measuring this magnetic flux density the magnetic flux density in the direction along the y-axis (axis connecting the magnetic poles 4) of the detector indicated in FIG. 3 is measured as a distribution in the direction along the x-axis (axis connecting the detecting electrodes 2). FIG. 4 shows a result thus obtained. As indicated there, the magnetic flux density varies within the piping and also the detected magnitude of the flow rate signal E given by Eq. (1) varies within the piping.
Furthermore, the flow rate signal detected by the detecting electrode 2 varies, even if products of the magnetic flux density B and the flow speed v are equal, depending on the position within the piping. In this case, the degree, with which the flow rate signal produced at different locations within the piping is detected, is represented by a weighting factor W given by: ##EQU1##
On the other hand the weighting factor W(x) for the magnitude of the signal produced in the x-direction by the magnetic field in the y-direction can be given by: ##EQU2##
Putting d=1, Eq. (3) can be rewritten as follows: ##EQU3## The result obtained by calculating this equation, inserting values of x and y for various locations therein, is shown in FIG. 5. This figure is identical to that published in JIS B 7554 stated above. In the figure only the half above the x-axis is shown. As can be seen from the figure, the weighting factor is very great at the neighborhood of the detecting electrodes and therefore the flow rate signal is produced mainly by fluid flowing at the neighborhood of the inner surface of the piping.
FIG. 6 shows variations of the flow rate .DELTA.Q flowing through a ring-shaped portion, whose radius varies, calculated by using the following formula; EQU .DELTA.Q=2.pi..times.r.times..DELTA.r.times.v (5)
where
As can be seen from FIG. 6, the flow rate .DELTA.Q varies proportionally to the radius.
By the prior art detector described above the magnitude of the flow rate passing through different portions detected as the flow rate signal is determined, depending on the product B.times.W of the magnetic flux density distribution and the weighting factor indicated in FIG. 5.
In the case where the magnetic flux density distribution is uniform as indicated in FIG. 7, the product B.times.W can be represented by a bar graph as indicated in FIG. 8. As can be seen from this graph the flow rate signal detected by the electrodes is determined approximately by the flow rate at the neighborhood of the inner surface of the piping. For this reason, inspite of the fact that most of the fluid flows at the central portion of the piping, the flow rate at the central portion is almost not detected as the flow rate signal, which means that the real flow rate is not measured with fidelity. Furthermore turbulence is easily produced in the flow in the neighborhood of the inner surface of the piping and the flow rate signal is apt to be influenced by noises due to this turbulence. In addition it is apt to be influenced by noises due to dirtiness of the surface of the electrodes.
FIGS. 9 and 10 show variations of the magnetic flux density distribution and the product B.times.W, respectively, with respect to the radial position in the detector indicated in FIG. 3. In this case also, it can be understood that the flow rate signal doesn't represent the flow rate at different portions with fidelity, although the relation is more or less improved in comparison with the case in FIGS. 7 and 8. Furthermore there is another problem that the flow rate signal is apt to be influenced by noises due to turbulence at the inner surface of the piping.